Multivibrator hold-off circuit



Oct. 30, 1962 J. R. KOBBE MULTIVIBRATOR HOLD-OFF CIRCUIT Original FiledFeb. 21

E m L R m m w N MR WE mm w TT A A .3 K w mm. R I N N E H m 0 J H K w BUnited States Patent 3,061,788 MULTIVIBRATOR HQLD-OFF CIRCUIT John R.Kobbe, Beaverton, 0reg., assignor to Tektronix, Inc., Portland, Oreg., acorporation of Oregon Original application Feb. 21, 1955, Ser. No.489,614, now Patent No. 2,853,609, dated Sept. 23, 1958. Divided andthis application Sept. 17, 1958, Ser. No. 761,570 7 Claims. (Ci.328-496) This invention relates to trigger circuits, and relatesparticularly to a novel method and circuit arrangement for use with amultivibrator to prevent actuation of the latter through prematuretriggering by random or other trigger signals during a desired cycle ofmultivibrator action.

The present application is a division of a copending application SerialNo. 489,614, filed February 21, 1955, filed as a joint application of R.L. Ropiequet and the present applicant and now Patent No. 2,863,609granted September 23, 1958, for Multivibrator Hold-Off Circuit.

In circuitry provided, for example, in cathode ray Oscilloscopes for thegeneration of a sawtooth waveform time base, it is sometimes thepractice to provide a multivibrator of the direct coupled type which hasno timing circuit components, and therefore the recovery time of themultivibrator is necessarily quite short. Accordingly, in order toprevent faulty operation it is required that means he provided toprevent the multivibrator from being triggered before the sawtoothgenerator has returned to its quiescent state, i.e., before the sweepcapacitor is completely discharged. Similarly, mono-stablemultivibrators provide recovery times shorter than the recovery of thesawtooth generator, and therefore the same precaution must be taken.

One means for achieving this result is to utilize the recovery waveformof the multivibrator or the sawtooth waveform of a sweep generator todisconnect the multivibrator from the trigger source. However, each ofthese waveforms has a region of voltage and time change near the end ofits recovery period in which the arrival of a trigger signal is capableof causing premature re-triggering of the multivibrator. Furthermore,this method does not control the operation of the multivibrator andtherefore, although it may be effective in preventing prematuretriggering by external signals, it cannot prevent premature operation ofthe multivibrator when the latter is in a free-running condition.

Another means for achieving this result involves the circuitry of themultivibrator in arrangements which prevent the multivibrator fromrecovering until a definite time has elapsed from the termination of thesawtooth sweep. At this time the multivibrator is then permitted torecover, but since the circuit elements of the multivibrator havecertain characteristic recovery times themselves, it frequently occursthat a trigger pulse will be impressed upon the multivibrator to triggerthe latter before complete recovery has been achieved.

It is the primary object of the present invention to provide a methodand hold-off circuit by means of which premature of otherwiseundesirable trigger signals are prevented from triggering amultivibrator during the desired cycle of operation of saidmultivibrator by positive means not subject to the characteristics andto change in the characteristics of the circuit components of themultivibrator.

Another important object of this invention is to provide a hold-offcircuit which is adjustable, whereby to provide for variable periods ofdelay before permitting a trigger signal to re-trigger themultivibrator.

A further important object of this invention is the provision of ahold-off circuit which is adjustable in such ice manner that themultivibrator may be reset by means of external triggers which may becontrolled automatically or manually, as may be desired.

Still another important object of this invention is the provision in amultivibrator-triggered sweep circuit of a hold-off circuit whichemploys the output sweep waveform to control the stability of themultivibrator.

A further important object of this invention is the provision of ahold-off circuit which functions to control the stability of amultivibrator and to prevent for a predetermined time its beingtriggered, by maintaining the control element thereof at a level ofpotential which is incapable of eifecting said triggering.

A still further object of the present invention is the provision of ahold-ofl circuit of the class described which is faithful in operationand is of simplified construction for economical manufacture.

These and other objects and advantages of the present invention willappear from the following detailed description taken in connection withthe accompanying drawing, in which:

FIG. 1 is a schematic diagram of one form of holdoif circuit embodyingthe features of the present invention; and

FIG. 2 is a graph showing the triggering waveform for the multivibratorand illustrating the operation of the hold-off circuit of FIG. 1.

Stated broadly, the method and hold-off circuit of this invention isassociated with a multivibrator whose triggering is effected at apredetermined level of potential applied to a suitable control element,and involves the control of said potential in such manner as to delaythe return of said control element to the triggering level for apredetermined time after reversion of the multivibrator.

Referring to FIG. 1 of the drawings, the multivibrator including thetubes V10 and V11 controls the operation of a sweep generator includingthe tubes V12, V13, V14 and V15, the output waveform of which is fedback to the multivibrator input to revert the multivibrator and is alsodelayed to function as a hold-off waveform. The source of triggersignals is connected to terminal 101 which is connected through adifferentiating network comprising capacitance 102 and resistance 103 tothe grid 104 of multivibrator tube V10. The time constant of thedifferentiating network is short, in order to limit the size of triggersignals. Grid 104 is connected through resistance 103 to the commoncathode connection of V17, V18 and V19 discussed hereinafter, saidcathode connection being returned through resistance 105 to a negativepotential, such as the 150 volts indicated.

The cathode 106 of tube V10 is conected to the cathode 107 ofmultivibrator tube V11, and these cathodes are returned to a negativepotential through resistance 108. The plate 109 of tube V10 is connectedthrough resistance 110 to a positive potential, such as the 100 voltsindicated, and also through the parallel combination of capacitance 111and resistance 112 to the grid 113 of tube V11. This grid is returned tothe negative potential indicated through resistance 114. The screen grid115 is connected to the positive potential indicated.

The plate 116 is connected to a positive potential through resistance117, and it is also connected to ground through resistance 118. Thecathodes of tubes V10 and V11 are suiiiciently negative to enable theplate of tube V11 to drop below ground.

The plate 116 of tube V11 is also connected to the plates 119, 120 ofthe respective disconnect diodes V12 and V13. The cathode 121 of diodeV12 is connected to the grid 122 of the sawtooth generator tube V14,said grid being returned to the negative potential indicated more fullyhereinafter. V adjust the grid voltage of cathode follower tube V19,

through resistance 123. The plate 124 of tube V14 is returned to thepositive potential indicated through resistance 125. e

The plate 124 of tube V14 is also connected through neon glow tube 126and resistance 127 to the grid 128 of cathode follower tube V15, saidgrid being returned to the negative potential indicated throughresistance 129. Capacitance 130 shunts the neon tube 126 and resistance127 to preserve the high frequency gain to grid 123. The cathode 131 oftube V15 is connected to the cathode 132 of diode V13. The cathode 131is also returned to the negative potential indicated through resistance133. Timing capacitor 135 is connected between the cathode of cathodefollower tube V15 and grid 122 of tube V14, as indicated.

The grid 136 of cathode follower tube V16 is con nected to the tap onpotentiometer resistance 133, while the cathode 137 is connected to thegrid 1:38 of cathode follower tube V17; The cathode 137 is returned tothe negative potential indicated through resistance 139, and

to ground through capacitance 140. The cathode 141 of cathode followerV17 is connected to the cathode 142 of cathode follower tube V18, bothcathodes being returned to the negative potential indicated throughresistance 105. The cathode 145 of tube V19 is also connected to thecathode 142 of tube V18. The grid 146 of tube V19 is connected to thetap on potentiometer resistance 147 which is connected between thenegative potential indicated and ground. The screen grid 148 isconnected to the positive potential indicated and the plate 149 isconnected through resistance 150 and switch 151 to the positivepotential indicated.

The plate 149 of cathode follower tube V19 is also connected to the grid143 of tube V18 through the parallel combination of resistance 152 andcapacitance 153. In this manner tubes V118 and V19 become a bistablemultiyibrator, for purposes described in detail hereinafter.

The plate 154 of amplifier tube V20 is also connected to the grid 143 ofcathode follower tube V18 through'the compensated voltage dividercomprising resistances 144 and 152 and capacitance 153. The screen grid155 of tube V20 is connected to a positive potential, as indicated, andthe cathode 156 is connected to the negative potential indicated throughthe parallel combination of resistance 157 and capacitance 158. Thecontrol grid 159 is connected to a negative potential of about -50volts, as indicated, through resistance 160. This grid is also connectedthrough capacitance 16 1 to terminal 162 to which is connected thesource of external trigger Fsignals 163. The plates of the tubes V19 andV20 normally operate at a negative potential with respect to ground,this being possible since the cathode of such V11 is held below cut-E,and when tube V11 is conducting, the cathode of tube V is heldsuificien-tly positive to hold V10 cut off.

To return the multivibrator to the rest state, with tube V10 conducting,there is required at the grid of tube V10 a positive voltage which ishigh enough *to cause plate current to flow. The positive voltage forthis purpose is supplied from the sweepwaveform, as explainedPotentiometer '147 functions to which, in turn, determines the voltagelevel of grid .104 during the rest state of tube V19. Various voltage V119, 120 to their quiescent state.

7 4 levels may be provided, as explained in detail hereinafter.

The general operation of the circuit described hereinbefore is asfollows: When a negative trigger signal 100 is applied to the grid 104,tube V10 is cut 011 and the resulting positive pulse 171 at plate 109 iscoupled to the grid 113 of tube V11, thereby raising said grid abovecut-01f and causing plate current to flow. The resulting rise of cathode107 also raises cathode 106, whereby tube V10 is removed still furtherfrom conduction. The negative step 172 from the plate 116 of tube V11 isapplied to the plates of the disconnect diodes V 12 and V13.

In the quiescent state between sweeps, the plates of diodes V12 and V13rest, for example, at 3.5 volts. Also the cathodes 121 and 132 of thetubes V12 and V13 are slightly negative relative to the plates 119 and120, respectively, so that the tubes V12 and V13 are conducting todischarge the capacitor 135 and to provide a direct current coupledfeedback loop from the plate 124 of the tube V14 through the neon tube126, the cathode follower V15 and the tubes V13 and V12 to the grid ofthe tube V14. The negative step to the diode plates lowers these platesbelow their cathodes, thus breaking the direct coupled feedback loopfrom plate 124 to grid 122 of tube V14. The capacitor 135 chargeslinearly and the resulting linear rise of the cathode 131 of cathodefollower tube V15 is used as the positivegoing portion 1'73 of a sweepsawtooth voltage which may be applied, for example, to the deflectionplates of a cathode ray tube oscilloscope (not shown), by connection atterminal 170. It will be understood that the values of timing capacitor135 and charging resistor 123 may be varied to cover a wide range ofsawtooth slopes. Further, the length of the sweep maybe adjusted bymeans of potentiometer resistance 133.

The cathode 131 of cathode follower tube V15 contimes to rise linearlyuntil a positive step from the sweep multivibrator tube V11 returns thedisconnect diode plates Such a positive step occurs when the grid 104 ofmultivibrator tube V10 'is brought positive enough to revert themultivibrator,

. and this is provided by the sawtooth sweep which is coupled backthrough cathode follower tubes V16 and V17 to the grid 104. The sweepdrops rapidly to its 'is delayed to produce a delayed sweep returnportion 174, whereby to retard the return of grid 104 to the quiescentlevel after the passage of the positive pulse. Thus, all trigger signalsare prevented from retriggering the multivibrator until all othercapacitances in the circuit have had time to reach their quiescentvoltage levels. The size of capacitor may be varied so that morerecovery time is permitted for slower sweeps and the least necessaryrecovery time is allowed for the faster sweeps. a

' The operation of the sweep multivibrator is controlled in both of itsstable states by the potential at grid 104. Over a certain region of itscontrol the grid exhibits hysteresis, and the hysteresis region isdefined with respect to waveform A, FIG. 2, by the lower limit 176 andthe upper limit 177. Within this region, tube V10 is conducting when theregion is approached from above the upper limit and tube V10 is cut offwhen the region is approached from below the lower limit. Thus, duringthe time of the positive-going portion 173 of the sweep tube V10 is cut.011 and during the delayed time of the negative-going return portion1.74 applied to the grid of tube V10, .such tube is conducting.

In order for the multivibrator to be cont-rolled by the sawtoothwaveform applied to grid 104, the magnitude of this waveform isselected, by proper values of circuit components, to be capable ofextending across these hysteresis limits. This is indicated in FIG. 2 bywaveform A, wherein the sawtooth waveform 173 is shown to originatebelow the lower hysteresis limit 176 at the cut-01f bias level 178 oftube V17 and to terminate slightly above the upper limit 177. Underthese conditions the sweep circuit is free running as explained below.The original negative-going return portion of this sweep waveform isshown in dotted line to return to this originating level. The delayedreturn 174 applied to the grid 104 of the tube V triggers themultivibrator, i.e., to render V11 conducting, when it crosses the lowerhysteresis limit 176. l

The delay between triggering of the multivibrator and the start of thegrid rise is due to the time required for the sawtooth sweep to risefrom the quiescent level 178 to the more positive voltage level 179 ofthe cathode 145 of tube V19.

Four modes of trigger hold-oil operation are provided by the circuitshown in FIG. 1. Two of these modes obtain with switch 151 in theposition identified as Sweep Normal. In this position, the plates oftube V19 and V are disconnected from the positive voltage source, andhence the plate of tube V19 drops to the potential of the l50 voltsupply. Simultaneously, the grid 143 of tube V18 also drops to thisnegative potential and is therefore cut off permanently. Screen currentfurnishes the cathode current of tube V19, and therefore the stabilitycontrol potentiometer 147 may be adjusted over its range to control thevoltage applied to grid 104 of tube V10.

In one mode of operation, this control 147 is adjusted to set the gridvoltage of tube V10 just below the lower hysteresis limit 176, asindicated in waveform A in FIG. 2. In this mode of operation the sweepcircuit is freerunning and requires no trigger signal 100, since thesawtooth sweep functions on its negative-going return automatically torevert the multivibrator and initiate another sweep. However, a triggersignal 100' arriving as the voltage of the grid 104 approaches the lowerhysteresis limit 176 will still trigger the sweep.

A second mode of operation obtains with switch 151 in the positionidentified as Sweep Normal, when the stability control potentiometer 147is adjusted to set the quiescent grid voltage of tube V10 just above thelower hysteresis limit 176. As indicated by waveform B in FIG. 2, thedelayed negative-going portion 174 of the sawtooth sweep returns to aquiescent level above the lower hysteresis limit, and thus requires atrigger signal 100 to intercept this lower limit to trigger the sweepmultivibrator. Thus, a subsequent sawtooth sweep is produced only upontriggering of the multivibrator by a negative trigger signal 100.

A third mode of operation occurs with switch 151 in the positionidentified as Sweep Delayed, and this mode is illustrated by waveform Cin FIG. 2. Voltage is now supplied to the plates of tubes V19 and V20,with tube V19 functioning with tube V18 to form a bistablemultivibrator. In the quiescent state, tube V18 conducts and holds thecommon cathode voltage so high that tube V19 is cut off. A positivedifferentiated trigger pulse 163 applied to grid 159 of tube V20produces a negative pulse at plate 154 which is coupled to the grid 143of tube V18. Thus, this tube is cut off and tube V19 is caused toconduct.

In the stable state wherein tube V18 is conducting said tube determinesthe voltage level of the common cathode circuit of cathodes 142 and 145,and this level 180 is high enough above the lower hysteresis limit 176to hold off tube V10 from being returned to a conducting state, even inthe presence of the negative trigger 100. Upon triggering of thehold-ofl? multivibrator by an external ter tube controls the commoncathode voltage level, and

6 this level 181 is lower than in the first state. Thus, by adjustmentof potentiometer 147 the resulting grid voltage of tube V10 may beplaced at or below the lower hysteresis limit 176 and the sweepmultivibrator will be triggered upon triggering of the hold-offmultivibrator by the external trigger signal 163. At the end of thepositive-going sweep, the hold-off cathode follower tube V17 raises thecommon cathode level of cathodes 14-2, 145 momentarily, therebyreverting the hold-off multivibrator with tube V18 conducting and tubeV19 cut off.

It is to be noted here that the potential of cathodes 142 and 145 iscontrolled by tube V17 until the sweep waveform has recovered completelyand is maintained above the conduction level of tube V19 during thattime. Accordingly, no external trigger signal 163 can revert thehold-off multivibrator prematurely.

A fourth mode of operation obtains with switch 151 in the positionidentified as Sweep Delayed and illustrated by waveform D in FIG. 2. Bysetting the cathode level by means of the stability potentiometer 147slightly above the lower hysteresis limit 176, the sweep will not betriggered while tube V19 is conducting, but the grid 104 of tube V10will be placed close enough to triggering that a negative trigger pulsefrom a trigger source will trigger the sweep multivibrator and initiatea sweep.

Thus, in the first mode of operation the delayed return portion 173 ofthe sawtooth sweep which is fed back to grid 10-1 operates recurrentlyto trigger the sweep multivibrator and any trigger signal 100 arrivingwhen such portion is approaching the lower hysteresis limit 1'76 willstill trigger the sweep multivibrator. In the second mode of operationthe sweep multivibrator is triggered by a negative trigger signal 100,after the circuit components have returned to their quiescent levels, asafforded by the delayed return portion 174 of the sawtooth sweep fedback to grid 104. In the third mode of operation the positive triggerpulse 163 actually controls the triggering of the sweep multivibrator.In the fourth mode the positive trigger pulse 163 functions to revertthe hold-off multivibrator to cause conduction of tube V19. The voltageon grid 104 is set close enough to triggering that the next negativetrigger pulse 100 will trigger a sweep multivibrator. The fourth mode ofoperation, when used in connection with a cathode ray tube oscilloscope,permits the display of delayed signals without jitter, even if thesignal itself jitters.

In the circuit illustrated in the drawings and described hereinbefore,triggering of the sweep multivibrator is eifected at a predeterminedlevel'of potential applied to a control element and the return of thiscontrol element to said triggering level after reversion of themultivibrator is controlled by a delayed hold-ofi signal which isinitiated by the signal produced by reversion of the multivibrator. Thehold-oif signal is either mixed with a trigger signal which will returnthe control element to triggering level, or the voltage level of thehold-off signal at its termination is adjusted to return the controlelement to the triggering potential of the sweep multivibrator. Thus,the hold-off circuit functions to control the stability of themultivibrator, to prevent improper operation as well as prematuretriggering.

It will be apparent to those skilled in the art that many modificationsand changes may be made in the illustrated arrangements described indetail hereinbefore without departing from the scope and spirit of thisinvention. For example, transistors may be substituted for the vacuumtubes, if desired, and they are intended to be included in the termelectron discharge device as employed in the appended claims.

Also the hold-off circuits and method of the present invention areapplicable to multivibrators of various types. In any case it isrequired only that triggering of the multivibrator be controlled by apredetermined level of potential applied to a suitable control elementof the multivibrator, such as the cathode, control grid or suppressorgrid of one of the multivibrator vacuum tubes. Further, it will beapparent that when differentiated trigger signals are employed totrigger the multivibrator, they need not be applied to the same elementthat carries the control potential, it being required only that thetrigger signals be applied to an element which is so associated with thecontrol element as to efiect mixing of the trigger signals and thehold-elf signal. Accordingly, it is to be understood that the foregoingdescription is merely illustrative and is not to be considered in alimiting sense.

Having now described my invention and the manner in which the same maybe used, what I claim as new and desire to secure by Letters Patent is:

1. A voltage generator circuit comprising:

multivibrator means having first and second stable i states forproviding a first control voltage when in said first state and a secondcontrol voltage when in said second state,

voltage generator means responsive to said control voltages forproducing an output voltage which varies from an initial value toward asecond value when themultivibrator means changes from said first stateto said second state and which rapidly returns to said initial valuewhen the multivibrator means changes back to said first state, 7

feedback circuit means for feeding said output voltage back to themultivibrator means to cause the multivibrator means to change back tosaid first state when said output voltage reaches said second value,

and time delay means in said feedback circuit means for preventing themultivibrator means from again changing from said first state to saidsecond state for a time delay after said output voltage has returned tosaid initial value.

2. A voltage generator circuit comprising:

multivibrator means having first and second stable states for providinga first control voltage when in said first state and a second controlvoltage when in said second state,

voltage generator means responsive to said control voltages forproducing an output voltage which varies linearly from an initial valuetoward a sec- 1 rapidly returns to said'initial value when themultivibrator means changes back to said first state, 7 trigger signalinput means for delivering trigger, signals to said multivibrator tocause said multivibrator to change from said first state to said secondstate, 7 feedback circuit means for feeding said output voltage back tothe multivibrator means to cause the multivibrator means to change backto said first state.

multivibrator means having first and second stable 7 states forproviding a first control voltage when said multivibrator means is insaidfirst state and a second control voltage when said multivibratormeans is in said second state,

voltage generator means responsive to said control' voltages forproducing an output voltage which varies linearly from an initial valuetoward a second value when the multivibrator means changes from saidfirst state to said second state and which rapidly returns to saidinitial value when said multivibrator means changes back to said firststate, voltage mixing means associated wtih said multivibrator means,trigger signal input means for delivering trigger voltages to saidmixing means to trigger said multivibrator means to cause it to changefrom said first state to said second state, feedback circuit means fordelivering said output volt.- age to said mixing means in opposition tosaid trigger signals to cause said multivibrator means to change back tosaid first state when said output voltage reaches said second value, andtime delay means in said feedback circuit means to prevent said triggersignals from again triggering said multivibra-tor means for a time delayafter said output voltage reaches said initial value. 4, A voltagegenerator circuit comprising: multivibrator means having first andsecond stable states for providing a first control voltage when saidmultivibrator means is in said first state and a second control voltagewhen said multivibrator means is in said second state, voltage generatormeans responsive to said control voltages for producing an outputvoltage which varies linearly from an initial value toward a secondvalue when the multivibrator means changes from said first state to saidsecond state and which rapidly returns to said initial value when saidmultivibrator means changes back to said first state, voltage mixingmeans associated with said mul-tivibrator means, trigger signal inputmeans for delivering trigger voltages to said mixing means to triggersaid multivibrator means to cause it to change from said first state tosaid second state, a direct current feedback circuit means fordelivering said output voltage to said mixing means 'in opposition tosaid trigger signals to cause said multivibrator means to change back tosaid first state 'when said output voltage reaches said second value,time delay means in said feedback circuit means to prevent said triggersignals from again triggering said multivibrator means for a time delayafter said output voltage reaches said initial value, andiadjustablevoltage means for delivering a direct current adjustable voltage to saidmixing means to 'setthe voltage of the trigger signal required totrigger said multivibrator means including an adjustable voltage whichwill cause triggering of said multi- =vibrator means after said timedelay upon return of said output voltage to said initial value. 5. Avoltage generator circuit comprising: multivihrator means having firstand second stable states for providing a first control voltage when saidmultivibrator means is in said first state and a second control voltagewhen said multivibrator means is in said second'statc, voltage generatormeans responsive to said control voltages for producing an outputvoltage which varies linearly from an initial value toward a secondvalue when the multivibrator means changes from said first state to saidsecond state and which rapidly returns to said initial value when saidmultivibrator means changes back to said first state, voltage mixingmeans associated with said multivibrator means, trigger signal inputmeans for delivering trigger volt- ;ages tosaid mixing means to triggersaid multivi- .brat or means to cause it to change from said first stateto said second state, direct current feedback circuit means fordelivering said output voltage to said mixing means in opposition tosaid trigger signals to cause said multivibrator means to change back tosaid first state when said output voltage reaches said second value,

and time delay means in said feedback circuit means to prevent saidtrigger signals from again triggering said multivibrator means for atime delay after said output voltage reaches said initial value,

second multivibrator means having .two steady states and supplying acontrol potential to said mixing means preventing triggering of thefirst mentioned multivibrator means when said second multivibrator is inone of its states and enabling said triggering when said secondmultivibrator means is in the other of the states,

means for supplying trigger signals to said second multivibrator meansfor causing said second multivibrator means to change from said onestate to said other state,

and connection means between said feedback circuit and said secondmultivibrator means to cause said second multivibrator means to changeback to its one state when said output voltage reaches said secondvalue.

6. A voltage generator circuit comprising:

multivibrator means having first and second stable states for providinga first control voltage when said multivibrator means is in said firststate and a second control voltage when said multivibrator means is insaid second state,

voltage generator means responsive to said control voltages forproducing an output voltage which varies linearly from an initial valuetoward a second value when the multivibrator means changes from saidfirst state to said second state and which rapidly returns to saidinitial value when said multivibrator means changes back to said firststate,

voltage mixing means associated with said multivibrator means,

trigger signal input means for delivering trigger voltages to saidmixing means to trigger said multivibrator means to cause it to changefrom said first state to said second state,

direct current feedback circuit means for delivering said output voltageto said mixing means in opposition to said trigger signals to cause saidmultivibrator means to change back to said first state when said outputvoltage reaches said second value,

and time delay means in said feedback circuit means to prevent saidtrigger signals from again triggering said multivibrator means for atime delay after said output voltage reaches said initial value,

second multivibrator means having two steady states and supplying acontrol potential to said mixing means preventing triggering of thefirst mentioned multivibrator means when said second multivibrator is inone of its states and enabling said triggering when said secondmultivibrator means is in the other of the states,

means for supplying trigger signals to said second multivibrator meansfor causing said second multivibrator means to change from said onestate to said other state,

connection means between said feedback circuit and said secondmultivibrator means to cause said second multivibrator means to changeback to its one state when said output voltages reaches said secondvalue,

and adjustable voltage means associated with said second multivibratormeans for delivering a direct ourrent adjustable voltage to said mixingmeans to set the voltage of the trigger signal required to trigger saidfirst mentioned multivibrator means including an adjustable voltagewhich will cause triggering of said first mentioned multivibrator meansafter said time delay upon return of said output voltage to said initialvalue.

7. A voltage generator circuit comprising:

first multivibrator means having two stable states and having an inputportion and also having an output portion for producing a gatingvoltage,

voltage generator means responsive to said gating voltage from saidoutput portion for producing a varying voltage which gradually changesfrom a first predetermined voltage when said multivibrator means changesfrom one of its states to its other state and for causing a rapid returnof said varying voltage to said first predetermined voltage when saidmultivibrator means changes back to said one state,

first input means for delivering input trigger pulses to the inputportion of said multivibrator means to cause said multivibrator means tochange from said one state to said other state,

circuit means including a cathode follower tube having a cathode loadresistance for feeding back the resulting gradually varying voltageproduced by said voltage generator means to said input portion of saidmultivibrator means for causing said multivibrator means to change fromsaid other state back to said one state when said gradually varyingvoltage reaches a second predetermined voltage,

said multivibrator means having a recovery period after changing back tosaid one state and said voltage generator means having a recovery periodwhen said varying voltage returns to said first predetermined voltage,

and time delay means in said circuit means including a capacitor alsoproviding a cathode load for said tube for preventing sai-d triggerpulses from causing said multivibrator means again changing to saidother state until after said recovery periods,

second multivibrator means having two stable states and being connectedto said circuit means so as to be changed from a second state to a firststate after said recovery periods for also preventing said triggerpulses from causing said first multivibrator from again changing to saidother state,

second input means for delivering trigger pulses to said secondmultivibrator for causing said second multivibrator to thereafter changeback to said second state to enable trigger pulses to cause said firstmultivibrator means to change to said other state.

References Cited in the file of this patent UNITED STATES PATENTS2,557,770 Scoles June 19, 1951 2,562,295 Chance July 3 1, 1951 2,577,475Miller Dec. 4, 1951 2,644,887 Wolfe July 7, 1953 2,661,421 Talamini Dec.1, 1953 2,688,079 Wachtell Aug. 31, 1954 2,748,272 Schrock May 29, 19562,764,681 Howell Sept. 25, 1956 2,769,905 Ropiequet Nov. 6, 1956 OTHERREFERENCES Millman and Taub: McGraw-Hill, pages 164-172.

